Image sensor chips, which include front side illumination (FSI) image sensor chips and Backside Illumination (BSI) image sensor chips, are widely used in applications such as cameras. In the formation of image sensor chips, image sensors (such as photo diodes) and logic circuits are formed on a silicon substrate (or a wafer), followed by the formation of an interconnect structure on a front side of the wafer. In the front side image sensor chips, color filters and micro-lenses are formed over the interconnector structure. In the formation of the BSI image sensor chips, after the formation of the interconnect structure, the wafer is thinned, and backside structures such as color filters and micro-lenses are formed on the backside of the silicon substrate. When the image sensor chips are used, light is projected on the image sensors, in which the light is converted into electrical signals.
The image sensors in the image sensor chips generate electrical signals in response to the stimulation by photons. Quantum efficiency (QE) of a photosensitive device measures a percentage of photons hitting a device's photoreactive surface that produce charge carriers.